The optical properties of electrochromic materials change in response to electrically driven changes in oxidation state. Thus, when an applied voltage from an external power supply causes electrons to flow to (reduction) or from (oxidation) an electrochromic material, its transmittance properties change. In order to maintain charge neutrality, a charge balancing flow of ions in the electrochromic device is needed. By enabling the required electron and ion flows to occur, an electrochromic device utilizes reversible oxidation and reduction reactions to achieve optical switching.
Conventional electrochromic devices comprise at least one thin film of a persistent electrochromic material, i.e., a material which, in response to application of an electric field of given polarity, changes from a high-transmittance, non-absorbing state to a low-transmittance, absorbing or reflecting state. Since the degree of optical modulation is directly proportional to the current flow induced by the applied voltage, electrochromic devices demonstrate light transmission tunability between high-transmittance and low-transmittance states. In addition, these devices exhibit long-term retention of a chosen optical state, requiring no power consumption to maintain that optical state. Optical switching occurs when an electric field of reversed polarity is applied.
To facilitate the aforementioned ion and electron flows, an electrochromic film which is both an ionic and electronic conductor is in ion-conductive contact, preferably direct physical contact, with an ion-conducting material layer. The ion-conducting material may be inorganic or organic, solid, liquid or gel, and is preferably an organic polymer. The electrochromic film(s) and ion-conductive material are disposed between two electrodes, forming a laminated cell.
When the electrode adjacent to the electrochromic film is the cathode, application of an electric field causes darkening of the film. Reversing the polarity causes electrochromic switching, and the film reverts to its high-transmittance state. Typically, an electrochromic film such as tungsten oxide is deposited on a substrate coated with an electroconductive film such as tin oxide or indium tin oxide to form one electrode. The counter electrode is typically a similar tin oxide or indium tin oxide coated substrate.
Since an electrochromic device may be modeled as a non-linear passive device having an impedance dominated by a capacitive component, the amount of charge transferred to an electrochromic device is typically controlled by utilizing current sources and current sinks.
Unfortunately, present control methodologies tend to be directed towards ease of use and accuracy, but not both. Therefore it is seen to be desirable to provide a method and apparatus facilitating ease of use and accuracy within the context of an electrochromic control system.